Mobility Support in IPv6 Charles E. Perkins David B. Johnson T. J. Watson Research Center Computer Science Department IBM Corporation Carnegie Mellon University Hawthorne, NY 10532 Pittsburgh, PA 15213 [email protected] [email protected] Abstract Without specific support for mobility in IPv6, packets destined to a mobile node would not be able to reach it while the mobile node IP version 6 (IPv6) is being designed within the IETF as a re- is away from its home IP subnet, since as in IPv4, routing is based placement for the current version of the IP protocol used in the on the network prefix in a packet’s destination IP address. In order Internet (IPv4). We have designed protocol enhancements for IPv6. to continue communication in spite of its movement, a mobile node known as Mobile IPv6, that allow transparent routing of IPv6 pack- could change its IP address each time it moves to a new IP subnet, ets to mobile nodes, taking advantage of the opportunities made but the mobile node would then not be able to maintain transport possible by the design of a new version of IP. In Mobile IPv6, each and higher-layer connections when it changes location. Mobility mobile node is always identified by its home address, regardless of support in IPv6 is particularly important, since mobile computers its current point of attachment to the Internet. While away from are likely to account for a majority or at least a substantial fraction its home IP subnet, a mobile node is also associated with a care-of of the population of the Internet during the lifetime of IPv6. address, which indicates the mobile node’s current location. Mo- IPv6 is derived from IPv4 and is in many ways similar to it. bile IPv6 enables any IPv6 node to learn and cache the care-of As such, the IETF Mobile IP Working Group’s current protocol address associated with a mobile node’s home address, and then design [22] for mobility of IPv4 nodes could be adapted for use in to send packets destined for the mobile node directly to it at this IPv6, with only the straightforward changes needed to accommodate care-of address using an IPv6 Routing header. differences between IPv4 and IPv6 such as the size of addresses. However, the development of IPv6 presents a rare opportunity, in that there is no existing installed base of IPv6 hosts or routers with 1 Introduction which we must be compatible, and in that the design of IPv6 may still be adjusted to account for the few special needs of mobile nodes. In this paper, we describe the design of a new protocol for trans- This paper, therefore, considers how IPv6 can most naturally fulfill parent routing of IPv6 packets to mobile IPv6 nodes operating in the support requirements for mobile nodes. the Internet [ 131. IP is the protocol which provides packet routing Each mobile node is assigned a (permanent) IP address in the and delivery services for the Internet, and IP version 6 (IPv6) [8] same way as any other node, and this IP address is known as the is a new version of IP intended to replace the current version of mobile node’s home address. A mobile node’s home address re- IP (IPv4) [26]. IPv6 is in the final stages of design within several mains unchanged regardless of where the node is attached to the working groups of the Internet Engineering Task Force (IETF) [ 11, Internet. The IP subnet indicated by this home address is the mo- 121, the principle standards development body for the Internet. bile node’s home subner. and standard IP routing mechanisms will deliver packets destined to a mobile node’s home address only to the mobile node’s home subnet. A mobile node is simply any node that David Johnson was supported in part by the National Science Foundation under CAREER Award NCR-9502725. by the Air Force Materiel Command may change its point of attachment from one IP subnet to another, (AFMC) and ARPA under contract numberF196828-93-C-0193, and by the while continuing to be addressed by its home address. Any node AT&T Foundation under a Special Purpose Grant in Science and Engineer- with which a mobile node is communicating we refer to here as a ing. The views and conclusions contained here me those of the authors and correspondent node. which itself may be either mobile or stationary. should not be interpreted as necessarily representing the official policies or A mobile node’s current location while away from home is endorsements, either express or implied, of NSF, AFMC, ARPA, the AT&T Foundation, IBM, CMU. or the U.S. Government. known as its care-of address, which is a globally-routable address acquired by the mobile node through IPv6 address autoconfigura- Permissionto make digital/hard copies of all or pati of this material for tion in the foreign subner being visited by it. The association of a personal or classroom use ia granted without fee provided that the copies mobile node’s home address with a care-of address, along with the are not made or distributed for profit or commercial advantage, the copy- remaining lifetime of that association, is known as a binding. right notice, the title of the publication and its date appear, and notice is given that copyright is by permission of the ACM, Inc. To copy otherwise, While away from its home subnet. a router on the mobile node’s to republish, to post on servers or lo redistribute to lists, requires specific home subnet known as its home agent maintains a record of the permission and/or fee. current binding of the mobile node. The home agent then intercepts MOBICOM 96, Rye NY USA Q 1996 ACM 0-89791-872-X/96/1 1 .X3.50 27 operation of a correspondent node communicating with a mobile Correspondent Packets from mobile Mobile node node node, and Seclion 8 describes the operation of a home agent. Slecu- node routed directly to rity issues are discussed in Section 9, and in Section 10. we present correspondent node a4 conclusions. 2 Overview of IPv6 Packets to mobile node In this section. we outline some of the basic characteristics of IPver- routed indirectly sion 6 (IPv6) that are particularly relevant to our mobility protocol. through home agent The most visible difference is that IPv6 addresses are all 128, bits \ long, instead of 32 bits long as in IPv4. Within this huge address space, a tiny )art is reserved for all current IPv4 addresses, and another tiny part is reserved for the Link-Local addresses, which Home agent are not routable but which are guaranteed to be unique on a link (i.e., on a local network). Nodes on the same link can communicate Figure 1 Triangle routing with each other even without any routers. by using their Link-Local addresses. Nodes discover each other’s presence, as well as each other’s any packets on the home subnet addressed to the mobile node’s link-layer (i.e., MAC) addresses, by participating in the Neighbor home address and tunnels them to the mobile node at its current Discovery pro:ocol [ 191; IPv6 nodes also discover local routers and care-of address. This tunneling uses IPv6 encapsulation [6], and network prefixes by means of Neighbor Discovery. The IPv6 Neigh- the path followed by a packet while it is encapsulated is known bor Discovery protocol can be characterized as a much improved as a tunnel. Once a correspondent node has learned the mobile version of twl) IPv4 protocols, the Address Resolution Protocol node’s care-of address, it may cache it and route its own packets for (ARP) [23] and the ICMP Router Discovery Protocol [7]. the mobile node directly there using an IPv6 Routing header [8], IPv6 defines several kinds of extension headers, which may be bypassing the home agent completely. used to incluce additional information in the headers of an IPv6 The most important function needed to support mobility is the packet. The dt:fined IPv6 extension headers include: reliable and timely notification of a mobile node’s current care-of address to those other nodes that need it, in order to avoid the routing l Destination Options header, anomaly known as triangle rot&g, as illustrated in Figure 1. In l Hopby-Hop Options header, triangle routing, all packets sent to a mobile node must be routed l a Routing header, and first to the mobile node’s home subnet and then forwarded to the l an Authentication header. mobile node at its current location by its home agent; packets sent The Destination Options header may be included in a p.acket from a mobile node are not forwarded in this way (unless they to carry a sequence of one or more options to be processed only are destined to another mobile node), leading to this “triangular” when the pac:cet arrives at the final destination node. Similarly, combination of the two routes used for all communication between the Hop-by-Hop Options header may be included to carry a se- these two nodes. quence of one or more options, but these options are processed by Triangle routing, because of its poor route selection, has many every intermediate router which receives and forwards the packet attendant problems, including as well as by the final destination node. In IPv4, every 11’ op- l increased impact of possible network partitions, tion [26] is treated essentially as a Hop-by-Hop option and thus causes performance degradation because of processing needed at l increased load on the network, and every intermediate router, whether it pertains to that router or only l increased delay in delivering packets.